Process of treating steel



Patented Nov. 3, 1936 UNITED STATES PATENT OFFICE No Drawing. Application June 14, 1932, Serial No. 617,243

10 Claims.

steel. The method may conveniently be illustrated as applied to the manufacture of automobile bump ers.

The steels which are at present used for the manufacture of automobile bumpers are either high carbon steels or special alloy steels such as chromo molybdenum, manganese vanadium and silica manganese alloys. Steels of this char acter are expensive to manufacture, and the use of same increases considerably the cost of manufacture of articles made from the same. Steels of this kind are employed however because it has been impossible heretofore to produce a low carbon steel having the characteristics required in the manufacture of articles of this character.

One object of the present invention is to provide a process of treatment of steel by which a low carbon steel may be produced having higher degrees of elasticity, tensile strength, ductility and toughness than the low carbon steels heretofore manufactured.

With the above and other objects in view, the invention consists of the process embodying the novel and improved features hereinafter described and particularly pointed out in the claims, the advantages of which will be readily understood and appreciated by those skilled in the art.

In making a bumper by the novel process of the present invention, a steel strip of the desired cross-sectional dimensions and of the length required to make a single bumper or a number of bumpers of the desired design is employed. This strip may be substantially straight or slightly or considerably curved, or of any other desired form. The strip however is preferably maintained in a relatively simple form until the finishing operations are performed on the same so that the surfaces will be readily accessible to the finishing tools. The strip may be formed of untempered or unhardened stock of low carbon steel composition and is preferably from A to of an inch in thickness. Highly satisfactory results have been obtained with steel stock containing from .30% to 33% of carbon, this steel being known in the art as knife-back steel. Steel as high in carbon as 40% may be employed in the present process. The finishing operations to prepare the surface of the strip for a plating of nickel or other plating material or for the reception of any other coating may be performed when the strip is in substantially straight or other suitable bar form.

Before the surfaces of the strip are finished,

This invention relates to a method of treating the strip is placed in a suitable pickle to remove the scale. This pickle may consist of any suitable pickling solution, such as the sulphuric acid pickling solution commonly used in pickling hot rolled steels to remove the scale. After remaining in the pickling solution a sufficient length of time to effectively remove the scale, the bar or strip is removed from the solution, is washed in an alkaline solution to neutralize the acid of the pickle and is then washed in water to remove the alkali. The bar or strip is then ready for the finishing operation. a

y In finishing the strip or bar, the strip is first rolled cold in a rolling mill to smooth out or iron out certain irregularities in the surface of the steel, the rolling operation also changing certain of the characteristics of the steel. The strip may be given any number of passes between the rolls and may be given any reduction or compression in each of said passes in order to produce 20 the desired results. In the rolling operation both of the wider faces of the strip or the flats are rolled or smoothed out at the same time and the sides or edge surfaces of the strip may be engaged by side rollers to roll out these surfaces at the same time. These rolling operations give a smooth finish to thesurfaces of the strip and enable the grinding of the surfaces with the two coarser grades of emery employed in prior operations to be eliminated. Also by rolling out the 30 surface of the strip which is to form the back surface in the finished bumper, this surface is rendered much smoother than in prior processes so that, in nickel plating, the back surface presents a better finish. Also smoothing out this back surface renders it much more resistant to the formation of rust after plating. This is also true of the edge surfaces of the strip, which as stated above, may also be smoothed out in the rolling operation.

After the surfaces of the strip have been rolled in the manner described above, certain surfaces may be subjected to further finishing operations to prepare these surfaces of the strip for a plating of nickel or other plating material or for the reception of a coating of japan, enamel or other coating material. These finishing operations may be performed when the strip is in a substantially straight or other suitable bar form.

In finishing certain surfaces of the strip, the 5 strip may be subjected to the action of rotary grinding wheels graded to produce the proper finishes upon the surfaces of the bumper. Both the back and front faces of the strip may be ground, although where it is desired to reduce the cost, one face only of the strip which is to form the outer face of the finished bumper is ground. Both edges of the strip may be ground or polished simultaneously by passing the strip between rotary grinding or polishing wheels arranged in opposed relation. As stated above however the grinding and polishing of both the lateral surfaces of the strip remove more of the metal than is desirable and greatly increases the cost of manufacture on account of the large number of grinding wheels worn out in this manner. Where the strip is passed through a rolling mill before the final grinding and polishing operations the rolling of the inner surfaces of the strip gives this surface a sufficient finish to meet the practi cal requirements.

Preferably instead of grinding and polishing the surfaces of the strip while the strip is in substantially straight or other bar form, the strip is rolled while cold into the form of a ring before the grinding and polishing operations are performed. These operations may be performed more quickly and efiiciently when the strip is in ring form. The expressions ring or ring form are to be understood as not necessarily referring to a perfect circular ring, but may include other ring shapes which are rotatable to present different portions of the ring successively to the action of the finishing tools.

The strip may be rolled out to give a smooth finish to the surfaces thereof and the strip may be rolled into ring form in one operation by passing the strip through a combined rolling and bending machine having a suitable arrangement of rollers. The untempered and unhardened condition of the stock enables the same readily to be plastically formed or molded permanently into a ring formation and also enables the surfaces readily to be rolled out in the manner described as preliminary steps in the surface finishing operations.

The strip having been rolled into ring form, the ends of the strip are welded together, preferably by an electric welding operation, and the flash formed in the welding operation is then removed in any suitable manner. The rolling operation will usually result in the formation of a slightly fiat portion at each end of the strip. After the ends of the strip are welded together, the ring is preferably placed in an expanding or in a shrinking machine which reduces the ring to a substantially perfect circular form, the fiat portion at each side of the weld being removed in this operation.

The grinding and polishing of the ring preparatory to the plating may be performed substantially in the manner described in my co-pending appication, Serial No. 71,833 filed November 27, 192

The surfaces of the steel may be completely finished in the rolling operations and the subsequent grinding of the surfaces may be omitted. As stated above, the steel may be given any number of passes between the rolls and may be given any reduction in order to produce the desired results. The strip may be given a reduction of 25% or more in its thickness or width as compared with its original thickness or width. The rolling operation not only produces a smooth finish on the surfaces of the strip but reduces the strip to a more nearly uniform gauge which is highly important in the manufacture of certain articles. The rolling also increases the hardness of the steel.

After the finishing operations have been performed on the strip; the strip is cut into a plurality of bars or ring sections of the length required for the bumper, bumper bar or other article to be produced.

The steel is preferably heated by immersing the same in a bath of fused salts having a melting point and a volatilization point sufficiently high to secure the desired degree of heat while the bath remains in a liquid condition. This bath material is of such a character. that it will have substantially no chemical action upon the steel during its relatively short period of immersion in the bath and that it will adhere to the surface of the metal and protect the same from oxidation when the metal is removed from the bath. This salt bath may consist of any salts or mixtures of salts found suitable to heat the metal to the required temperature during a relatively short period without appreciable action on the metal and to protect the metal from oxidation when removed from the heating bath. Preferably however this bath consists of a mixture of 50% potassium chloride and 50% sodium chloride, a. bath of this kind having been found to produce very efficient results in actual practice. Each bar, ring, or ring section after the finishing operation, is immersed in the heating bath which is heated to a temperature around 1550 F., depending upon the carbon content of the steel. Temperatures from 1450 F. to 1650 F. have been found to be effective to reduce the steel to the desired condition. The bar, ring, or ring section is heated in this salt bath usually from two to three minutes. The immersion of the metal for this length of time in the salt bath is sufiicient to heat the metal to the proper temperature for bending into the desired bumper form and for thereafter hardening the metal, and does not give the bath material sufficient time to attack the metal and destroy the fine finish placed upon it in the polishingor other finishing operations. This time element in the heating step is also highly important in imparting the desired properties to the steel. The heating should be short of producing an annealed structure and destroying the advantageous qualities produced by the cold rolling. The time during which the steel should be heated depends to a certain degree on the extent of the cold rolling, the thickness of the strip and the carbon in its composition. With a steel composition containing .30 to .40% carbon, the steel is preferably heated in the salt bath from two to three minutes.

When the steel has been immersed in the heating bath for the length of time required to heat the metal to the proper temperature for.

bending and hardening, it is removed from the heating bath with a coating of the bath material adhering thereto and bent into the desired shape. The ring, ring section or bar is preferably bent into the desired form by inserting the same into a suitable forming mechanism. When the metal is heated in ring form, the ring may be placed in a forming mechanism of the character disclosed in my Patent No. 1,917,267, dated July 11, 1933, and bent into shape by the action of this mechanism.

While the metal is still at a high temperature, and preferably while still held by the forming mechanism, it is immersed in a suitable quenching bath. A quenching bath is selected which will quickly dissolve the coating of the mixture of sodium and. potassium chlorides from the heating bath adhering to the surface of the metal and which will harden the metal uniformly to the desired degree. The quenching bath employed should also be such as to prevent the formation of oxide 'on the metal surfaces while cooling and also to provide an adherent coating for protecting the surface of the metal from rapid oxidation when it is removed from the bath.

Quenching baths suitable to produce these results may be made up of various substances or mixtures. In actual practice, a bath of the following proportions has been found to produce highly efficient and satisfactory results:

Water"; 500 gallons Sodium chloride 150 pounds Potassium chloride 150 pounds Lime 50 pounds When the steel is plunged into this quenching bath, the said bath instantly dissolves the salt coating of the heating bath and the metal is cooled rapidly and uniformly. The solid particles of lime in suspension in the bath eliminate the formation of gas pockets and spotty irregularities or the formation of soft spots. The solid constituents of this bath left upon the steel by the evaporation of the water when the steel is removed from the bath protects the metal from rapid oxidation.

When a day's time is to elapse between the removal of the bumper from the quenching bath and the placing of the bumper in the plating bath, the quenching bath is thoroughly agitated to keep the ingredients of the bath in suspension. This causes a relatively heavy coating of the bath constituents to be formed upon the metal when it is removed from the bath to protect the surface from rapid oxidation.

When the steel is to be plated within an hour or two after it is removed from the quenching bath, the quenching bath is not agitated but the precipitates are allowed to settle to the bottom of the bath.

The coating of the quenching bath materials is largely soluble in water and may be easily removed from the steel by washing the same in water.

After the formed bumper has been immersed in the quenching bath, it is removed from the bath preferably while still heated from 150 F. to 200 F. It is then placed on a rack and allowed to cool. the bath, the bumper is suificiently warm to dry the metal quickly, leaving a deposit from the quenching bath upon the surfaces to protect the metal from rust. During the quenching operation, sizzling takes place while the metal is cooling, which gradually diminishes and thus constitutes an indication as to the proper time for drawing the steel from the bath. The strip may be left in the bath until practically cold if desired.

The surfaces of the strip when taken from the quenching bath, are bright and without any oxide formation or pitting, and will have a high degree of hardness, ductility, tensile strength and elasticity.

During the quenching operation the bumper is subjected to heavy internal stresses and should then be held from distortion by suitable mechanism. The bumper may be held on the forming mechanism during the quenching operation to hold the same from distortion.

The immersion of the metal in the heating and quenching baths removes all traces of the grease which may remain on the metal after the final finishing of the same with emery paste. This is Thus, upon being removed from.

highly important since a film of grease upon the surface of the steel to which the plating material is to be applied may ruin the plating operation.

When a relatively thin strip of steel stock having a highly polished surface is heated to hardening temperature in a salt bath of the above character which protects the polished surface from oxidation, and is then plunged in an aqueous quenching bath such as that described in this application, a mottled appearance will be imparted to the polished surface of the steel. This mottled effect is reproduced on the surface of the metal plating which follows the somewhat irregular surface of the steel formed by heating and quenching the steel in the manner described. When the strip of steel is passed one or more times through a rolling mill at a preliminary stage in the manufacture of a bumper, it is found that this mottled effect produced upon plunging the steel into the aqueous quenching bath is much more pronounced. This may be due to the compression and hardening of the surfaces of the steel produced by the action of the rollers of the rolling mill upon the same. This mottled appearance on the nickel surface is somewhat similar in appearance to hammered silver except that the re fleeting surfaces, or facets, are not nearly so pronounced. As the treatment of the steel is varied to impart diminishing degrees of hardness to the same, these facets become smaller and smaller, until with a treatment which imparts a relatively low degree of hardnessto the steel, they practically disappear. These individual reflecting surfaces reflecting the light in many different directions as compared with the smooth or plane surfaces formed under ordinary conditions, thus give a very distinctive appearance to the nickel surface of the bumper.

In the rolling operation the stock is rolled between hard and preferably highly finished rolls and may be reduced in dimension by 25% of its original dimension, or more. A high finish may be given to the steel in the rolling operation so that certain or all of the grinding and polishing operations may be eliminated. The rolling operation also reduces the stock to a much more nearly uniform gauge than has been done in Elastic limit Tensile strength 180,000 to 185,000 pounds per sq. 200,000 to 215,000 pounds per sq.

in. in.

Elongation in 2 inches Reduction in area 10% to 11% 30% to 40%.

Hardness Brinell Fracture 402 to 410 Full cup.-

til

'I'hus the steel has an elasticity and tensile strength considerably better than-has been produced in a steel containing a carbon content of 1%, and its ductility and toughness as expressed by reduction in area, are very much greater. This high reduction in area indicates high shock or impact resistance and high fatigue values which render the steel particularly adaptable for use in automobile springs and bumpers.

As stated above the steel strip may be removed from the quenching bath while still hot and allowed to cool in the air, or it may be allowed to remain in the bath until it has cooled substantially to the temperature of the bath. The strip P may then be heated in an open hearth furnace or in an oil bath to a temperature of about 600". An oil bath is preferable since no oxide is formed. By heating the steel to this temperature for about half an hour, a spring temper is given to the steel of about 375 to 450 Brinell.

Instead of a quenching bath of the composition given above, a quenching bath may be employed consisting of brine to which has been added 3% to 5% of sodium cyanide and enough lime to exceed the solubility, leaving the excess lime in suspension.

This application constitutes a continuationin-part of applicant's copending application Serial No. 173,797, filed March 8, 1927 on Process of making bumpers and the like for automobiles.

Having explained the nature and object of the invention and having specifically described the man er in which the invention is preferably applied, what is claimed is:

1. A process of treating steel comprising cold rolling a piece of unhardened and untempered steel, heating the steel to a temperature of about 1450 F. to 1650 F. for less than five minutes in a non-oxidizing heating medium, and quenching the steel in an aqueous quenching medium.

2. A process of treating steel comprising cold rolling a. piece of unhardened and untempered steel, heating the steel to a temperature of about 1450 F. to 1650 F. for a period of less than five minutes in a bath of fused salts, and quenching the steel in a bath containing a salt of one of the alkali metals.

3. A process of treating steel comprising cold rolling a piece of unhardened and untempered steel, heating the steel to a temperature of about 1450 F. to 1650" F. for a period of less than five minutes in a bath of fused salts, and quenching the steel in a bath containing a salt of one of the alkali metals and lime.

4. A process of treating steel comprising cold working a piece of unhardened and untempered steel, heating the steel to a temperature of about 1450" F. to 1650 F. for a period of less than five minutes in a non-oxidizing heating medium, and plunging the steel into a quenching medium.

5. A process of treating steel comprising cold rolling a piece of unhardened and untempered steel, heating the steel to a temperature of about 1550 F. for a period of less than five minutes in a non-oxidizing heating medium, and plunging the steel into a quenching medium.

6.. A process of treating steel comprising cold rolling a piece of unhardened and untempered steel, heating the steel to a temperature of about 1450 F. to 1650 F. for a period of two to three minutes in a bath of fused salts, and quenching the steel in an aqueous quenching bath.

7. A process of treating steel comprising cold working a piece of unhardened and untempered low carbon steel to give the steel a finer crystal or grain structure as well as to initially harden the steel, heating the steel rapidly to a temperature of about 1450" F. to 1650" F. in a heated bath for a sufiiciently short period of time so as not to change the crystal or grain structure obtained during the cold working operation such as less than five minutes for steel strips up to approximately three eighths of an inch in thickness, and quickly thereafter plunging the steel into a quenching bath to harden the steel.

8. A process of treating steel comprising cold working a piece of unhardened and untempered low carbon steel to give the steel a finer crystal or grain structure as well as to initially harden the steel, heating the steel rapidly to a temperature of about 1450 F. to 1600 F. in a heated bath for a sufiiciently short period of time so as not to change the crystal or grain structure obtained during the cold working operation and quickly thereafter plunging the steel into a quenching bath to harden the steel.

9. A process of treating steel comprising cold working a piece of unhardened and untempered low carbon steel having a carbon content of about .30 to 33% to initially harden the steel, heating the steel rapidly to a temperature around 1550 F. in a non-oxidizing heating medium for a sufliciently short period of time so as not to change the characteristics of the steel obtained during the cold working operation and quickly thereafter chilling the steel to harden the same.

10. The process of treating unhardened and untempered steel which exhibits both pearlite and ferrite crystals, comprising cold working said steel, heating the steel rapidly to a temperature in the vicinity of the Ac; point in a non-oxidizing heating medium for a sufficiently short period of time so as not to change the characteristics of the steel obtained during the cold working operation and quickly thereafter quenching the steel to harden the same.

GEORGE ALBERT LYON. 

